The amount of data generated in companies or by individuals is rapidly increasing year by year. Especially in companies, there are demands to cut down costs related to media for storing confidential data and other data safely or to cut down management costs.
Some storage systems are provided with a deduplication function or a compression function as functions for cutting down the actual amount of stored data itself.
First, the deduplication function of a storage system generally detects duplicated data in file units or specific data length units, and reduces the amount of data by not storing duplicated data. Such deduplication function is considered to be effective for storage systems used for backup purposes based on such characteristics.
Further, the compression function of a storage system generally divides the data within the volume into given lengths, and compresses the data within each given length so as to compress the whole data within the volume to thereby cut down the capacity. In the present specification, the division of data in a given length is called a compression unit.
The merit of the function for reducing the amount of stored data itself is, of course, the reduction of media costs due to the reduction of the amount of data. On the other hand, the demerit of such function is the deterioration of access performance to the data. Especially in the compression function, when a data read request targeting compressed data is received, the data must be decompressed via compression units, according to which the data read performance is deteriorated due to overhead of decompression processing. Further, when an update request targeting the compressed data is received, the whole data must be decompressed to have the update data overwritten thereto, and the data must be compressed again via compression processing for storage. Therefore, the update performance is deteriorated due to the overhead of the decompression processing and the compression processing.